When a cylinder head and a cylinder block of an internal combustion engine are secured in the prior art, as shown in FIG. 34, after situating a metal gasket formed with a bead 53 for surrounding a combustion chamber opening 42 between the gap of a cylinder head 45 and a cylinder block 46, it is bolted with a bolt 40 at a predetermined bolt fastening part 41. At this time, the bead 53 is elastically deformed by a fastening force of the bolt 40, and the elastically restoring force and the elastic restoration at the time of the elastic deformation form an elastic sealing line between the gap between the cylinder head 45 and the cylinder block 46 and thereby sealing the gap.
However, although the cylinder head 45 is bolted at each fastening part 41 with the uniform fastening force as shown in FIG. 33, the whole gap of the cylinder head 45 and the cylinder block 46 can not be secured uniformly and such gap changes variously according to the relation with the position of the bolt fastening part 41.
This is because the larger the distance from the fastening part 41 is, the lower the fastening force of the bolt 40 becomes. As a result of that, as the distance from the bolt fastening part, 41 becomes larger, the gap between the cylinder head 45 and the cylinder block 46 becomes larger, thereby deteriorating the sealing pressure of the bead.
In particular, in the multi-cylinder engines arranging a plurality of the combustion chamber openings 42 as shown in FIG. 33, such gap clearance between the cylinder head 45 and the cylinder block 46 gradually increases in order of a gap portion extending from the bolt fastening part 41 to a part around the combustion chamber opening 42 (A-A'-A" cross-sectional part of FIG. 33), a gap portion between the bolt fastening parts 41 and 41 (B-B' cross-sectional part of FIG. 33), and a gap portion between the combustion chamber openings 42 and 42 (C-C' cross-sectional part of FIG. 33). This leads to considerable blowing-off of burned gas between the adjacent combustion chamber openings.
Further, the surfaces of gasket between the cylinder head 45 and the cylinder block 46 are beaten due to the repetition of the increase and decrease of the gap clearance by the explosion gas pressure in the combustion chamber opening 42. As a result, the gasket is easily apt to cause failure deformation not only between the bolt fastening parts 41 and 41 but also the bead near the bolt fastening part 41, thereby deteriorating the sealing pressure of the gap between the cylinder head 45 and the cylinder block 46. This results in serious problems such as an enhanced blowing-off of the gas and loosening of the fastening bolts 40.
Accordingly, in order to solve the above mentioned problems, in the conventional metal gasket, a supplementary plate 52 is provided against a base plate 51 in which a bead 53 is formed, as shown in FIG. 34.
The reference is made to the explanation about the conventional metal gasket based on FIGS. 35 (a) to (c). FIG. 35 (a) corresponds to a A-A'-A" cross-sectional part of FIG. 33 and is a cross-sectional view of a part around the combustion chamber opening of the metal gasket in the gap from the bolt fastening part 41 to the part around the combustion chamber opening 42. Hereinafter, similarly, FIG. 35 (b) corresponds to a B-B' cross-sectional part of FIG. 33 and is a cross-sectional view of a part around the combustion chamber opening of the metal gasket between the bolt fastening parts 41 and 41, and FIG. 35 (c) corresponds to a C-C' cross-sectional part of FIG. 33 and is a cross-sectional view of a part of the metal gasket between the combustion chamber openings 42 and 42.
In the FIGS. 35 (a) to (c), the reference numeral 51 is a base plate for forming a bead 53 in which the bead having an arc and convex shape in the cross-section is formed, and 52 is a supplementary plate. This supplementary plate 52 is provided on whole surface of the base plate 51 in the concave side of the bead 53 and is folded to the convex side of the bead 53 at an inner periphery end of a plane plate portion 51a to overlap to the surface of the plane plate portion 51a, thereby forming a portion 52a increased in thickness. Further, in FIG. 35 (c), when the distance between the combustion chamber openings 42 and 42 is narrow and it is not possible to form an independent bead 53 relative to each combustion chamber opening 42, two beads are connected to each other and merged in one bead 53 at the center point between the combustion chamber openings 42 and 42. Accordingly, the plane plate portions 51a and 51a are provided at both sides of the bead 53 and this results in the construction of overlapping the portions 52a and 52a increased in thickness under the plane plate portions 51a and 51a.
In the above mentioned construction, by controlling bead heights H51, H52, and H53 and the bead widths W51, W52, and W53 in accordance with the gap clearance between the cylinder head 45 and the cylinder block 46, the gap between the cylinder head 45 and the cylinder block 46 can be compensated. Further, by providing a portion 52a increased in thickness around the combustion chamber opening 42, it is possible to compensate the gap between the cylinder head 45 and the cylinder block 46 around the combustion chamber opening 42 in which the gap clearance, at the time of fastening the bolt, is large.
However, according to the above mentioned metal gasket, since the supplementary plate 52 is provided on whole surface of the base plate 51 in the concave side of the bead 53 and is folded at the side the combustion chamber opening 42 to overlap with the plane plate portion 51a, two inner and outer sealing lines 51f and 51j formed in the concave side of the bead 53 are overlapped by the supplymentary plate 52 and each of the increased amounts in thickness at the sealing lines 51f and 51j by the supplymentary plate 52 becomes equal. On the other hand, as for the bead heights H51, H52, and H53 projecting to the convex side of the bead 53, the deflective height is reduced for the thickness S51 of each portion 52a increased in thickness, thereby causing the problem of deteriorating the sealing pressure of the bead. In other words, the portion 52a increased in thickness provided to compensate the gap whose clearance is larger around the combustion chamber opening 42 than the other gap portion and causes the deterioration of the sealing pressure of the bead 53 and the sealing function by the bead.
Further, between the combustion chamber openings 42 having a largest gap clearance between the cylinder head 45 and the cylinder block 46, as shown in FIG. 35 (c), not only the deflective height of the bead 53 is reduced by the thickness S51 of both portions 52a and 52a increased in thickness but also both portions 52a and 52a increased in thickness tend to be in the state of floating between the gap, because these portions 52a and 52a locate in the convex side of the bead 53. That is, the more the bead height H53 of the bead 53 increases, the more both portions 52a and 52a are beaten by the repetition of the increase and decrease of the gap clearance due to the explosion gas pressure, thereby causing the breakage of both portions 52a and 52a and deteriorating the sealing function.
Even though both portions 52a and 52a increased in thickness are formed to be thicker than other parts of the portion 52a in FIGS. 35 (a) or (b), corresponding to the large gap clearance between the combustion chamber openings 42 and 42, such gap clearance changes in accordance with the variations in dimensions by each engine and the order of fastening the bolts even in the same type of engine. Accordingly, it may produce the reverse effect on the sealing function by thickening the portion 52a increased in thickness and it is also very difficult to process the portion 52a having a variable thickness.
Further, if failure deformation occurs in the bead 53 by beating due to the explosion gas pressure, the fastening bolt 40 may come loose. This causes considerable deterioration of the engine function.